1. Field of the Invention
The present invention relates to a process for making selected halopyridine-N-oxides by the oxidation of the corresponding halopyridine with peracetic acid generated in-situ. In particular, the present invention relates to a process for making 2-chloropyridine-N-oxide or 2-bromopyridine-N-oxide by the oxidation of the corresponding halopyridine with peracetic acid generated in-situ from acetic acid and H.sub.2 O.sub.2 while in the presence of a catalyst selected from the group consisting of H.sub.2 SO.sub.4, alkali metal bisulfates, ammonium bisulfate, and mixtures thereof.
2. Description of the Prior Art
2-Chloropyridine and 2-bromopyridine are chemical intermediates which may be converted to the sodium and zinc salts of pyridine-2-thiol-N-oxide. See U.S. Pat. Nos. 2,686,786 and 3,203,957, which issued to Shaw et al on Aug. 17, 1954, and Kirchner on Aug. 31, 1965, respectively. These compounds may also be converted to bis(2-pyridyl-1-oxide) disulfide. See U.S. Pat. Nos. 3,892,760 and 3,954,781, both of which issued to Hooks, Jr. anc Pitts on July 1, 1975, and May 4, 1976, respectively. All of these end products are excellent biocides and have been used in hair shampoos or skin cleansing preparations, or the like.
Because 2-chloropyridine-N-oxide is generally more economic to make, most of the work centered around improving its preparation. However, the procedures disclosed herein for making 2-chloropyridine-N-oxide may be practiced to convert 2-bromopyridine to its N-oxide with generally similar results.
In the past, 2-chloropyridine-N-oxide had been made from 2-chloropyridine by various methods. As shown in U.S. Pat. No. 2,951,844 which issued to Shermer on Sept. 6, 1960, 2-chloropyridine may be reacted with an aqueous peracetic acid solution in a mole ratio of 0.4 to 0.8 mole of peracetic acid per mole of 2-chloropyridine. The peracetic acid content of the aqueous solution may be preferably about 30 to 50 weight percent. This reaction may be shown by the following equation (A): ##STR1## Upon reaction completion, the reaction mixture may be then neutralized with a base to a pH of 5 to 8, converting the acetic acid by-product to an acetate salt, for example, by using an aqueous NaOH solution. The unreacted 2-chloropyridine is recovered from the reaction mixture by distillation. Shermer teaches that a convenient method for preparing the peracetic acid comprises mixing 70 parts by weight of glacial acetic acid with 30 parts by weight of H.sub.2 O.sub.2 in the presence of one part of sulfuric acid. The resulting mixture contains about 40% peracetic acid, 40% acetic acid, 15% water, and 5% H.sub.2 O.sub.2 (with less than 1% H.sub.2 SO.sub.4). See Col. 2, lines 5-11 of this patent.
This "Preformed Peracetic Acid Oxidation Route" as taught by Shermer has several disadvantages. Peracetic acid is relatively unstable which makes its storage a problem. Also, there are a limited number of suppliers which may make it relatively expensive. Further, since excess 2-chloropyridine is used as a starting material, low batch productivity results and greater reactor volumes are needed. Still further, because more total acid is used, more base is needed for neutralization and the effluent from the process will have a high biological oxygen demand (BOD).
Finger and Starr in "Aromatic Fluorine Compounds. IX.2-Fluoropyridines", J. Am. Chem. Soc., Vol. 81, pages 2674 and 2675 (1959) teach another "Preformed Peracetic Acid Oxidation Route". They reacted 2-chloropyridine with a commercial 40% by weight peracetic acid solution in the presence of extra acetic acid. The mole ratio of 2-chloropyridine to total acid was 1:6.9. This high molar ratio has the same disadvantages of Shermer, but, more so.
In U.S. Pat. No. 3,203,957, which issued to Kirchner on Aug. 31, 1965, 2-chloropyridine-N-oxide is made by oxidizing 2-chloropyridine with H.sub.2 O.sub.2 and maleic or phthalic anhydride at a temperature in the range from 30.degree. C. to about 90.degree. C. The mole ratio of H.sub.2 O.sub.2 to 2-chloropyridine may be from 0.5:1 to 1.2:1 and the mole mole ratio of maleic or phthalic anhydride to H.sub.2 O.sub.2 is at least 1:1. The reaction may be preferably carried out in the presence of an inert solvent simply for the purpose of facilitating physical handling of the reaction mixture. Kirchner also teaches that the reaction involves the in situ formation of monoperoxymaleic or monoperoxyphthalic acid.
This "Monoperoxy-Maleic or Phthalic Anhydride Oxidation Route" also has several disadvantages. First, the maleic and phthalic anhydride reactants are relatively more expensiVe that acetic acid. Also, the presence of the by-product sodium maleate or sodium phthalate after neutralization sometimes presents problems in making later end products.
For practical purposes, the process taught by Kirchner requires an inert solvent or excess 2-chloropyridine. The use of an inert solvent or excess 2-chlorpyridine contributes to lower batch productivity and the use of the former also may require a separation step. Furthermore, the use of maleic or phthalic anhydride as reactants may create greater environmental problems than when comparable amounts of acetic acid are used because of their greater molecular weights.
Besides the above-noted patented methods for making 2-chloropyridine-N-oxide, other processes have been described in the literature. R. F. Evans and H. C. Brown [J. Org. Chem., 27, 1329 (1962)]taught that 2-chloropyridine-N-oxide may be prepared by reacting 2-chloropyridine with glacial acetic acid and H.sub.2 O.sub.2 at 70.degree.-80.degree. C. for about 12 hours without the use of any catalyst. However, they employed an acetic acid to 2-chloropyridine mole ratio of about 10.5:1. Katritzky [J. Chem. Soc., 191 (1957)]also describes the making of 2-chloropyridine-N-oxide by reacting 2-chloropyridine with acetic acid and aqueous H.sub.2 O.sub.2 without any catalyst overnight at 80.degree. C. Again, he employed an acetic acid to 2-chloropyridine mole ratio of about 13.2:1. The use of these exhorbitant molar quantities of acetic acid and long reaction times does result in low batch productivity; require large amounts of NaOH for neutralization; and result in a high BOD in the waste water effluent from the process. This non-catalytic in situ technique is only of academic interest and has imited practical application.
Work was also carried out wherein 2-chloropyridine was oxidized with peracetic acid formed in situ by the reaction of acetic acid and H.sub.2 O.sub.2 in the presence of an acid cation exchange resin catalyst (i.e., a sulfonated copolymer of styrene and 8% divinyl-benzene). See Example VI of U.S. Pat. No. 3,203,957, which issued to Kirchner. The patent admits this technique for oxidizing 2-chloropyridine to 2-chloropyridine-N-oxide was inferior. It should be noted that this reaction resulted in the low consumption of H.sub.2 O.sub.2 (74.6%). This means the reaction rate will be relatively slow and methods for disposing the unreacted H.sub.2 O.sub.2 must be employed.
Non-published work has been carried out at Olin Corporation for making 2-chloropyridine-N-oxide by reacting 2-chloropyridine with acetic acid and H.sub.2 O.sub.2 in the presence of very small amounts of H.sub.2 SO.sub.4 (0.02 mole of H.sub.2 SO.sub.4 per 1.0 mole of 2-chloropyridine reactant). Furthermore, this work employed an excess of 2-chloropyridine over H.sub.2 O.sub.2 (1.0 mole: 0.6 mole). It should be noted that this work also resulted in the low consumption of H.sub.2 O.sub.2 (57%). The excess 2-chloropyridine contributed to low batch productivity.
In U.S. Pat. No. 3,047,579, which issued to Witman on July 31, 1962, 2-chloropyridine may be oxidized with H.sub.2 O.sub.2 to 2-chloropyridine-N-oxide in the presence of "unstable inorganic per-compounds of the acid-forming elements of groups VA, VIA, VIB, and VIII" of the periodic table (e.g., pertungstic acid) as catalysts. Witman also teaches that this type of catalyzed reaction may be most effectively carried out in a liquid phase reaction medium, using a lower aliphatic monocarboxylic acid such as glacial acetic acid. See Col. 5, line 44 to Col. 6, line 5 of this patent.
This "Tungsten Catalyzed Oxidation Route" also has some disadvantages. While higher batch productivity may be more consistently achieved than in the above-discussed preformed peracetic acid route, the expensive tungsten catalyst must be recovered from the reaction mixture for economic reasons. However, there may be some carry-over of the tungsten with the 2-chloropyridine-N-oxide product. This carry-over which is extremely difficult to prevent, may result in undesirably colored sodium or zinc salts of pyridine-2-thiol-N-oxide later made from this product.
In all, the conversion and selectivity of 2-chloropyridine to 2-chloropyridine-N-oxide with some of these prior art processes have not been appreciably high, especially in large-scale production modes. Therefore, there is a need to raise the conversion and selectivity of this reaction and similar reactions to lower the costs of producing the N-oxide products and products derived from them. Furthermore, the production of zinc pyridine-2-thiol-N-oxide from 2-chlorpyridine-N-oxide produced by these prior art processes has sometimes been associated with serious color problems (i.e., this zinc salt has been too dark) which prevent it from being used in certain shampoo formulations. As stated above, it is believed that these color problems are caused by the presence of by-products of the desired N-oxide product. Thus, there is also a need to produce N-oxide products which do not have an appreciable amount of by-products which effect undesirable colors to final products. Furthermore, as can be seen from the discussion above, higher batch productivity and reduced organic effluents are desired. The present invention presents a solution to all of these needs.